Abstract base class for tasks that run within a ForkJoinPool.
A ForkJoinTask is a thread-like entity that is much
lighter weight than a normal thread. Huge numbers of tasks and
subtasks may be hosted by a small number of actual threads in a
ForkJoinPool, at the price of some usage limitations.

A "main" ForkJoinTask begins execution when submitted
to a ForkJoinPool. Once started, it will usually in turn
start other subtasks. As indicated by the name of this class,
many programs using ForkJoinTask employ only methods
fork() and join(), or derivatives such as invokeAll. However, this class also
provides a number of other methods that can come into play in
advanced usages, as well as extension mechanics that allow
support of new forms of fork/join processing.

A ForkJoinTask is a lightweight form of Future.
The efficiency of ForkJoinTasks stems from a set of
restrictions (that are only partially statically enforceable)
reflecting their main use as computational tasks calculating pure
functions or operating on purely isolated objects. The primary
coordination mechanisms are fork(), that arranges
asynchronous execution, and join(), that doesn't proceed
until the task's result has been computed. Computations should
ideally avoid synchronized methods or blocks, and should
minimize other blocking synchronization apart from joining other
tasks or using synchronizers such as Phasers that are advertised to
cooperate with fork/join scheduling. Subdividable tasks should also
not perform blocking IO, and should ideally access variables that
are completely independent of those accessed by other running
tasks. These guidelines are loosely enforced by not permitting
checked exceptions such as IOExceptions to be
thrown. However, computations may still encounter unchecked
exceptions, that are rethrown to callers attempting to join
them. These exceptions may additionally include RejectedExecutionException stemming from internal resource
exhaustion, such as failure to allocate internal task
queues. Rethrown exceptions behave in the same way as regular
exceptions, but, when possible, contain stack traces (as displayed
for example using ex.printStackTrace()) of both the thread
that initiated the computation as well as the thread actually
encountering the exception; minimally only the latter.

It is possible to define and use ForkJoinTasks that may block,
but doing do requires three further considerations: (1) Completion
of few if any other tasks should be dependent on a task
that blocks on external synchronization or IO. Event-style async
tasks that are never joined (for example, those subclassing CountedCompleter) often fall into this category. (2) To minimize
resource impact, tasks should be small; ideally performing only the
(possibly) blocking action. (3) Unless the ForkJoinPool.ManagedBlocker API is used, or the number of possibly
blocked tasks is known to be less than the pool's ForkJoinPool.getParallelism() level, the pool cannot guarantee that
enough threads will be available to ensure progress or good
performance.

The primary method for awaiting completion and extracting
results of a task is join(), but there are several variants:
The Future.get() methods support interruptible and/or timed
waits for completion and report results using Future
conventions. Method invoke() is semantically
equivalent to fork(); join() but always attempts to begin
execution in the current thread. The "quiet" forms of
these methods do not extract results or report exceptions. These
may be useful when a set of tasks are being executed, and you need
to delay processing of results or exceptions until all complete.
Method invokeAll (available in multiple versions)
performs the most common form of parallel invocation: forking a set
of tasks and joining them all.

In the most typical usages, a fork-join pair act like a call
(fork) and return (join) from a parallel recursive function. As is
the case with other forms of recursive calls, returns (joins)
should be performed innermost-first. For example, a.fork();
b.fork(); b.join(); a.join(); is likely to be substantially more
efficient than joining a before b.

The execution status of tasks may be queried at several levels
of detail: isDone() is true if a task completed in any way
(including the case where a task was cancelled without executing);
isCompletedNormally() is true if a task completed without
cancellation or encountering an exception; isCancelled() is
true if the task was cancelled (in which case getException()
returns a CancellationException); and
isCompletedAbnormally() is true if a task was either
cancelled or encountered an exception, in which case getException() will return either the encountered exception or
CancellationException.

The ForkJoinTask class is not usually directly subclassed.
Instead, you subclass one of the abstract classes that support a
particular style of fork/join processing, typically RecursiveAction for most computations that do not return results,
RecursiveTask for those that do, and CountedCompleter for those in which completed actions trigger
other actions. Normally, a concrete ForkJoinTask subclass declares
fields comprising its parameters, established in a constructor, and
then defines a compute method that somehow uses the control
methods supplied by this base class. While these methods have
public access (to allow instances of different task
subclasses to call each other's methods), some of them may only be
called from within other ForkJoinTasks (as may be determined using
method inForkJoinPool()). Attempts to invoke them in other
contexts result in exceptions or errors, possibly including ClassCastException.

Method join() and its variants are appropriate for use
only when completion dependencies are acyclic; that is, the
parallel computation can be described as a directed acyclic graph
(DAG). Otherwise, executions may encounter a form of deadlock as
tasks cyclically wait for each other. However, this framework
supports other methods and techniques (for example the use of
Phaser, helpQuiesce(), and complete(V)) that
may be of use in constructing custom subclasses for problems that
are not statically structured as DAGs. To support such usages a
ForkJoinTask may be atomically tagged with a short
value using setForkJoinTaskTag(short) or compareAndSetForkJoinTaskTag(short, short) and checked using getForkJoinTaskTag(). The ForkJoinTask implementation does not use
these protected methods or tags for any purpose, but they
may be of use in the construction of specialized subclasses. For
example, parallel graph traversals can use the supplied methods to
avoid revisiting nodes/tasks that have already been processed.
(Method names for tagging are bulky in part to encourage definition
of methods that reflect their usage patterns.)

Most base support methods are final, to prevent
overriding of implementations that are intrinsically tied to the
underlying lightweight task scheduling framework. Developers
creating new basic styles of fork/join processing should minimally
implement protected methods exec(), setRawResult(V), and getRawResult(), while also introducing
an abstract computational method that can be implemented in its
subclasses, possibly relying on other protected methods
provided by this class.

ForkJoinTasks should perform relatively small amounts of
computation. Large tasks should be split into smaller subtasks,
usually via recursive decomposition. As a very rough rule of thumb,
a task should perform more than 100 and less than 10000 basic
computational steps, and should avoid indefinite looping. If tasks
are too big, then parallelism cannot improve throughput. If too
small, then memory and internal task maintenance overhead may
overwhelm processing.

This class provides adapt methods for Runnable
and Callable, that may be of use when mixing execution of
ForkJoinTasks with other kinds of tasks. When all tasks are
of this form, consider using a pool constructed in asyncMode.

ForkJoinTasks are Serializable, which enables them to be
used in extensions such as remote execution frameworks. It is
sensible to serialize tasks only before or after, but not during,
execution. Serialization is not relied on during execution itself.

Unschedules and returns, without executing, the next task
queued by the current thread but not yet executed, if one is
available, or if not available, a task that was forked by some
other thread, if available.

Methods inherited from class java.lang.Object

Constructor Detail

ForkJoinTask

Method Detail

fork

Arranges to asynchronously execute this task. While it is not
necessarily enforced, it is a usage error to fork a task more
than once unless it has completed and been reinitialized.
Subsequent modifications to the state of this task or any data
it operates on are not necessarily consistently observable by
any thread other than the one executing it unless preceded by a
call to join() or related methods, or a call to isDone() returning true.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Returns:

this, to simplify usage

join

Returns the result of the computation when it is
done. This method differs from get() in that
abnormal completion results in RuntimeException or
Error, not ExecutionException, and that
interrupts of the calling thread do not cause the
method to abruptly return by throwing InterruptedException.

invokeAll

Forks the given tasks, returning when isDone holds for
each task or an (unchecked) exception is encountered, in which
case the exception is rethrown. If more than one task
encounters an exception, then this method throws any one of
these exceptions. If any task encounters an exception, the
other may be cancelled. However, the execution status of
individual tasks is not guaranteed upon exceptional return. The
status of each task may be obtained using getException() and related methods to check if they have been
cancelled, completed normally or exceptionally, or left
unprocessed.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Parameters:

t1 - the first task

t2 - the second task

Throws:

java.lang.NullPointerException - if any task is null

invokeAll

Forks the given tasks, returning when isDone holds for
each task or an (unchecked) exception is encountered, in which
case the exception is rethrown. If more than one task
encounters an exception, then this method throws any one of
these exceptions. If any task encounters an exception, others
may be cancelled. However, the execution status of individual
tasks is not guaranteed upon exceptional return. The status of
each task may be obtained using getException() and
related methods to check if they have been cancelled, completed
normally or exceptionally, or left unprocessed.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Parameters:

tasks - the tasks

Throws:

java.lang.NullPointerException - if any task is null

invokeAll

Forks all tasks in the specified collection, returning when
isDone holds for each task or an (unchecked) exception
is encountered, in which case the exception is rethrown. If
more than one task encounters an exception, then this method
throws any one of these exceptions. If any task encounters an
exception, others may be cancelled. However, the execution
status of individual tasks is not guaranteed upon exceptional
return. The status of each task may be obtained using getException() and related methods to check if they have been
cancelled, completed normally or exceptionally, or left
unprocessed.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Parameters:

tasks - the collection of tasks

Returns:

the tasks argument, to simplify usage

Throws:

java.lang.NullPointerException - if tasks or any element are null

cancel

public boolean cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task. This attempt will
fail if the task has already completed or could not be
cancelled for some other reason. If successful, and this task
has not started when cancel is called, execution of
this task is suppressed. After this method returns
successfully, unless there is an intervening call to reinitialize(), subsequent calls to isCancelled(),
isDone(), and cancel will return true
and calls to join() and related methods will result in
CancellationException.

This method may be overridden in subclasses, but if so, must
still ensure that these properties hold. In particular, the
cancel method itself must not throw exceptions.

This method is designed to be invoked by other
tasks. To terminate the current task, you can just return or
throw an unchecked exception from its computation method, or
invoke completeExceptionally(java.lang.Throwable).

isCompletedNormally

Returns true if this task completed without throwing an
exception and was not cancelled.

Returns:

true if this task completed without throwing an
exception and was not cancelled

getException

public final java.lang.Throwable getException()

Returns the exception thrown by the base computation, or a
CancellationException if cancelled, or null if
none or if the method has not yet completed.

Returns:

the exception, or null if none

completeExceptionally

public void completeExceptionally(java.lang.Throwable ex)

Completes this task abnormally, and if not already aborted or
cancelled, causes it to throw the given exception upon
join and related operations. This method may be used
to induce exceptions in asynchronous tasks, or to force
completion of tasks that would not otherwise complete. Its use
in other situations is discouraged. This method is
overridable, but overridden versions must invoke super
implementation to maintain guarantees.

Parameters:

ex - the exception to throw. If this exception is not a
RuntimeException or Error, the actual exception
thrown will be a RuntimeException with cause ex.

complete

Completes this task, and if not already aborted or cancelled,
returning the given value as the result of subsequent
invocations of join and related operations. This method
may be used to provide results for asynchronous tasks, or to
provide alternative handling for tasks that would not otherwise
complete normally. Its use in other situations is
discouraged. This method is overridable, but overridden
versions must invoke super implementation to maintain
guarantees.

Parameters:

value - the result value for this task

quietlyComplete

public final void quietlyComplete()

Completes this task normally without setting a value. The most
recent value established by setRawResult(V) (or null by default) will be returned as the result of subsequent
invocations of join and related operations.

java.util.concurrent.CancellationException - if the computation was cancelled

java.util.concurrent.ExecutionException - if the computation threw an
exception

java.lang.InterruptedException - if the current thread is not a
member of a ForkJoinPool and was interrupted while waiting

java.util.concurrent.TimeoutException - if the wait timed out

quietlyJoin

public final void quietlyJoin()

Joins this task, without returning its result or throwing its
exception. This method may be useful when processing
collections of tasks when some have been cancelled or otherwise
known to have aborted.

quietlyInvoke

public final void quietlyInvoke()

Commences performing this task and awaits its completion if
necessary, without returning its result or throwing its
exception.

helpQuiesce

public static void helpQuiesce()

Possibly executes tasks until the pool hosting the current task
is quiescent. This method may
be of use in designs in which many tasks are forked, but none
are explicitly joined, instead executing them until all are
processed.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

reinitialize

public void reinitialize()

Resets the internal bookkeeping state of this task, allowing a
subsequent fork. This method allows repeated reuse of
this task, but only if reuse occurs when this task has either
never been forked, or has been forked, then completed and all
outstanding joins of this task have also completed. Effects
under any other usage conditions are not guaranteed.
This method may be useful when executing
pre-constructed trees of subtasks in loops.

Upon completion of this method, isDone() reports
false, and getException() reports null. However, the value returned by getRawResult is
unaffected. To clear this value, you can invoke setRawResult(null).

inForkJoinPool

Returns true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation.

Returns:

true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation,
or false otherwise

tryUnfork

public boolean tryUnfork()

Tries to unschedule this task for execution. This method will
typically succeed if this task is the most recently forked task
by the current thread, and has not commenced executing in
another thread. This method may be useful when arranging
alternative local processing of tasks that could have been, but
were not, stolen.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Returns:

true if unforked

getQueuedTaskCount

public static int getQueuedTaskCount()

Returns an estimate of the number of tasks that have been
forked by the current worker thread but not yet executed. This
value may be useful for heuristic decisions about whether to
fork other tasks.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Returns:

the number of tasks

getSurplusQueuedTaskCount

public static int getSurplusQueuedTaskCount()

Returns an estimate of how many more locally queued tasks are
held by the current worker thread than there are other worker
threads that might steal them. This value may be useful for
heuristic decisions about whether to fork other tasks. In many
usages of ForkJoinTasks, at steady state, each worker should
aim to maintain a small constant surplus (for example, 3) of
tasks, and to process computations locally if this threshold is
exceeded.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Returns:

the surplus number of tasks, which may be negative

getRawResult

Returns the result that would be returned by join(), even
if this task completed abnormally, or null if this task
is not known to have been completed. This method is designed
to aid debugging, as well as to support extensions. Its use in
any other context is discouraged.

setRawResult

Forces the given value to be returned as a result. This method
is designed to support extensions, and should not in general be
called otherwise.

Parameters:

value - the value

exec

protected abstract boolean exec()

Immediately performs the base action of this task and returns
true if, upon return from this method, this task is guaranteed
to have completed normally. This method may return false
otherwise, to indicate that this task is not necessarily
complete (or is not known to be complete), for example in
asynchronous actions that require explicit invocations of
completion methods. This method may also throw an (unchecked)
exception to indicate abnormal exit. This method is designed to
support extensions, and should not in general be called
otherwise.

Returns:

true if this task is known to have completed normally

peekNextLocalTask

Returns, but does not unschedule or execute, a task queued by
the current thread but not yet executed, if one is immediately
available. There is no guarantee that this task will actually
be polled or executed next. Conversely, this method may return
null even if a task exists but cannot be accessed without
contention with other threads. This method is designed
primarily to support extensions, and is unlikely to be useful
otherwise.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Returns:

the next task, or null if none are available

pollNextLocalTask

Unschedules and returns, without executing, the next task
queued by the current thread but not yet executed. This method
is designed primarily to support extensions, and is unlikely to
be useful otherwise.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Returns:

the next task, or null if none are available

pollTask

Unschedules and returns, without executing, the next task
queued by the current thread but not yet executed, if one is
available, or if not available, a task that was forked by some
other thread, if available. Availability may be transient, so a
null result does not necessarily imply quiescence
of the pool this task is operating in. This method is designed
primarily to support extensions, and is unlikely to be useful
otherwise.

This method may be invoked only from within ForkJoinPool computations (as may be determined using method
inForkJoinPool()). Attempts to invoke in other contexts
result in exceptions or errors, possibly including ClassCastException.

Returns:

a task, or null if none are available

getForkJoinTaskTag

public final short getForkJoinTaskTag()

Returns the tag for this task.

Returns:

the tag for this task

Since:

1.8

setForkJoinTaskTag

public final short setForkJoinTaskTag(short tag)

Atomically sets the tag value for this task.

Parameters:

tag - the tag value

Returns:

the previous value of the tag

Since:

1.8

compareAndSetForkJoinTaskTag

public final boolean compareAndSetForkJoinTaskTag(short e,
short tag)

Atomically conditionally sets the tag value for this task.
Among other applications, tags can be used as visit markers
in tasks operating on graphs, as in methods that check: if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))
before processing, otherwise exiting because the node has
already been visited.

Parameters:

e - the expected tag value

tag - the new tag value

Returns:

true if successful; i.e., the current value was
equal to e and is now tag.